1. Field of the Invention
The present invention relates to detector apparatus for laser light scattering photometers and, more particularly, to apparatus for receiving and detecting scattered and transmitted power issuing from a sample in a laser light scattering photometer and for providing an output indicative of the received radiant power irrespective of a spatial difference between the two beams.
2. Description of the Prior Art
Inhomogeneity in the polarizability of liquids and solids leads to the scattering of an incident beam of light. Since this inhomogeneity extends to molecular dimensions, it can be used to characterize the size, motion, and thermodynamic behavior of molecules. The inhomogeneity may be consequence of fluctuations in concentration or density or may simply result from interfaces between molecular aggregates of different polarizability or refractive index. Consequently, light scattering can be used to study solutions, dispersions, and surfaces.
The intensity of scattered radiant energy depends upon the wavelength of the incident radiant energy, the polarizability of the particles or molecules compared with that of the medium in which they are suspended, and on the size and concentration of the particles. It has also been found that the intensity of the radiant energy scattered in the forward direction by a single particle will be proportional to the square of its volume and independent of its shape if the particle is isotropic and if its dimensions are small compared to the wavelength of the incident radiant energy. Since the forward radiant energy scattered by a dispersion containing particles is greater as the individual particles become larger, the size of the particles may be determined from the intensity of the radiant energy scattered when the refractive indices of the particles and of the medium are known. In a similar manner, the scattering properties associated with particle size have been adapted to the determination of the molecular weights of large molecules.
Light scattering is potentially faster, more accurate, and applicable over a wider molecular-weight range than other techniques employing viscosity, osmometry, or gravitational sedimentation. However, several problems have plagued light scattering measurements. In the first instance, the solutions have to be clarified--freed of particles--a procedure that has sometimes required days of preparation. Secondly, large volumes of sample have been required.
The light scattering technique most frequently employed for molecular weight determination involves preparation of a Zimm plot. Extrapolation of the Zimm plot to zero angle and zero concentration permits the determination of the weight average molecular weight. Unfortunately, this extrapolation to zero angle is subject to large errors when the angular function is non-linear and when particles contaminate the sample. Molecular weights above a few million are particularly difficult to determine because of uncertainties in the extrapolation to zero angle.
These measurements would be facilitated and accuracy would be improved if they could be obtained directly at very low angles and concentrations. These problems have stimulated numerous efforts to develop instruments operating at small scattering angles and small sample volumes. Unfortunately, the minimum scattering angle of most commercial light scattering photometers is 20.degree.-30.degree.. Attempts to reduce the scattering angle in custom instruments have been limited to 10.degree.-15.degree. because background signals from unwanted reflections and particulate contaminants increase rapidly with a decrease in scattering angle.
One of the most recent developments in this rapidly evolving field is the measurement of light scattered at small forward angles using a laser as the source of illuminating radiant energy. The laser provides a narrow beam of intense radiant energy which is both monochromatic and coherent in nature. The narrow beam permits scattering measurement at small angles relative to the incident direction of the laser beam. The intensity of the radiant energy contained in the beam enables greater sensitivities than photometer instruments not employing the laser as a source.
Perhaps the greatest advantage of the use of a laser for molecular weight determination is in minimizing the clarification problem. Defraction of incident light scattered by foreign particles of a size comparable to the wavelength of light increases drastically as the scattering angle .theta. decreases. This problem is a major contributor to the hazard in extrapolating the Zimm plot to zero angle. However, a laser beam can be focused to a small diameter, achieving an extremely small scattering volume. Typical scattering volumes encountered heretofore were approximately 1 ml whereas this has been reduced to 10.sup.-.sup.4 to 10.sup.-.sup.6 ml in a laser photometer. As a result, the probability of a foreign particle residing within the scattering volume is proportionately reduced. Because of the high power density, any particle within the scattering volume scatters intensely and its presence is obvious.
The small sample volume required to fill the sample cell further facilitates sample clarification. Sample volumes about 1,000 times less than that of cells in conventional variable angle instruments have been possible. Obviously, with less sample to filter, the clarification can be accomplished in a shorter period of time, an important factor when performing kinetic measurements. The short-path cells also minimize problems of sample absorption. Furthermore, the cell design permits use of flowing filtered samples without allowing them to contact the air.
Another area benefited by the laser is micro-fluorescence spectroscopy. The monochromatic lines of the laser are useful for exciting fluorescence in a variety of materials and the fact that the beam from these lasers can be focused to a very small spot provides the potential for measurements concerning fluorescence of microscopically small samples. Another application related to fluorescence is the study of excitation and deexcitation processes of the triplet states in organic molecules.
A low angle laser light scattering photometer is described in an article entitled "Light Scattering Measurements on Liquids at Small Angles" by W. I. Kaye, A. J. Havlik, and J. B. McDaniel, Polymer Letters, Volume 9, pages 695-699 (1971). Improvements in this photometer are described in an article entitled "Low Angle Laser Light Scattering--Absolute Calibration" by W. I. Kaye and A. J. Havlik, Applied Optics, Volume 12, No. 3, pages 541-550 (March, 1973) and in an article entitled "Low-Angle Laser Light Scattering" by W. I. Kaye, Analytical Chemistry, Volume 45, No. 2, pages 221A-225A (February, 1973). These articles describe a low angle laser light scattering photometer including a helium-neon laser operating in the TEM.sub.00 mode, the rays from which are focused by a lens onto a sample confined between two thick silica windows and a black Teflon spacer. Certain of the rays scattered from the sample through an angle .theta., defined by an annulus, are focused by a relay lens onto a field stop. Rays passing through the field stop are focused by an objective lens onto the end-window of a photomultiplier detector. The output of the detector is proportional to the total radiant power falling thereon, P.sub..theta..
The primary laser beam, attenuated by suitable attenuators, is transmitted through the sample, in the direction of the incident beam, and is focused by the relay lens onto the field stop. These rays, having a radiant power P.sub.0, are then focused by the objective lens onto the end-window of the photomultiplier detector. The ratio P.sub..theta./P.sub.0 is utilized to determine the Rayleigh factor, R.sub..theta., which is then utilized to calculate the molecular weight.
Since P.sub..theta..perspectiveto.10.sup..sup.-9 P.sub.0, it is necessary that the photomultiplier detector have high sensitivity. Furthermore, since the curve of molecular weight versus concentration, which must be extrapolated to zero concentration, is a linear function only at very low concentrations, very low concentrations must be used, requiring high instrument sensitivity if the number of measurements are to be minimized. Rapid kinetic measurements also require high sensitivity.
The detector must exhibit spatial uniformity since the scattered beam and the transmitted beam are incident on the detector at different angles and are of different cross-sectional areas. However, problems have been encountered in the past in that existing photomultipliers generate different signal levels when two beams are incident directly thereon at different angles. Furthermore, the sensitivity (amperes per watt) of most photocathodes varies significantly with the position of the cathode illuminated.
One approach to the solution of this problem has been to enlarge the size of the beam since this has an integrating effect. However, this has been an undesirable solution in laser photometers where it is desirable to minimize the sample size and the scattering volume.
It has also been suggested to use an integrating sphere or a diffuser plate to spread the incident scattered and transmitted energy over the surface of the photomultiplier. Integrating spheres have been unacceptable in low angle instruments because of the resultant loss of sensitivity. Furthermore, instruments that use diffusers assume that the areas of the detected transmitted and scattered beams cancel each other. However, this situation prevails only if the illuminating beam is well collimated, the beam is of uniform intensity, the detector does not see past the edges of the illuminated sample, and the detector sensitivity is constant over the illuminated area of the photocathode. To insure these conditions, the optical system becomes very inefficient in the sense that few of the potentially available photons emitted by the source can be detected. Thus, this approach too has been unacceptable in low angle instruments.